Tsuna goes undercover
by Flexecutioner
Summary: What happens when a Mafia boss in training is assigned to a middle school to help with the assassination of his teacher. The catch? His teacher is a yellow octopus that can reach Mach speeds and he destroyed half the moon. Oh and they have to kill him by the end of the school year or else he'll blow destroy earth.
1. Chapter 1

"Class this is Sawada Tsunayoshi, he has moved here from Namimori and he will help you assassinate Korosensei." The class looked at the meek, doe-eyed, fluffy haired young man.

Tsuna bowed towards the class and said a polite "It's nice to meet you I look forward to having class with you." The girls swooned at his attitude and voice.

"You can have the seat right behind Nagisa." Tsuna nodded and sat in the empty seat behind a blue haired boy.

The lesson continued on with several assassination attempts with guns and anti sensei bullets, enough that Korosensei banned guns from the class room. Tsuna watched the class and took it all in stride as he was relatively used to this, though it was usually him getting the assassination attempts.

Tsuna kept his identity to himself and kept up with the fake story Reborn gave him. He still used his real name but almost nothing about his past was true, well except about the bullying but he hadn't told them that yet.

All they knew about their new classmate was that he was a transfer student, seemed just as dame as Nagisa when school first started, and that he looked weak and scrawny.

Reborn had been smart enough to realize that Tsuna would always be underestimated so he based most of Tsuna's tor-... training on keeping him slim and lithe that kept with his petite frame instead of bulky and too muscular.

Not to be confused with Tsuna having no muscles, he had plenty, Reborn just kept his muscles strong yet barely visible to keep up with the scrawny and weak facade.

Line

It was several weeks into the semester and Tsuna was fitting in better than he ever had. He was still bullied and looked down upon by some of his classmates and kids from the main building but he wasn't as bothered as before.

"Tsuna! Come eat lunch with us!" Nagisa yelled as he dragged Karma behind him to their favorite tree. Tsuna smiled and quickly made his way over to the pair.

"How's it going Nagisa? Karma?" Tsuna smiled politely as he opened his bento. The trio spent the rest of lunch talking, well Nagisa and Tsuna did Karma just hummed and entered his input whenever needed.

Back in class everything was going smoothly for once before Tsuna's hyper intuition kicked into high gear alerting him of immediate danger. Automatically Tsuna tensed up becoming more alert and his eyes turned their fiery orange hue signaling he was close to HDW Mode.

"Tsuna?" Korosensei asked as he saw the sudden change in his student. Tsuna looked to the window suddenly when he saw a glint of light from a tree quickly followed by a bullet grazing his unflinching cheek.

"Amateurs." Tsuna scoffed as he got up from his seat just in time for the windows to blow in and several armed men came charging through at him.

Tsuna dispatched each with ease even as his cheek bled freely making quite the intimidating sight for his classmates. Tsuna grabbed the last conscious man and slammed him to the floor quickly putting his knee on the mans chest.

"Who sent you? Who do you work for?" Tsuna hissed the fire on his forehead roared slightly as his eyes took on a bloodier tint.

"It was the Medici! They came to us and asked us to kill the Vongola tenth, no questions asked!" The man gasped trying to release the pressure from his chest. Tsuna sucked his teeth and sent a wave of highly concentrated flames through the mans body knocking him out instantly.

Tsuna looked up into the shocked faces of his classmates realizing the mistake he just made. He turned to Korosensei and said "Excuse me sensei I need to make a call." Then proceeded to jump out the window and to a tree several yards away.

"Chaos." Came the reply Tsuna always heard when calling his tutor.

"I just had another attempt on my life Reborn. I thought you said this was top secret." Tsuna was in his rare boss mode reserved for his guardians when they acted up and attempts on his life meaning this was no time to joke around.

"It was supposed to be. Apparently we have some cleaning house to do. Who ordered it and were you injured?" Came Reborns cold reply, Tsuna could hear things being moved in the background so he knew Reborn was already making preparations for either an attack on Medici or cleaning house.

"Medici. I got grazed but I'll be fine. It's superficial probably won't even need stitches." Reborn hummed.

"I'm sending backup immediately they'll be there by next week." He gave Tsuna no time to protest and ended the call, furious that his student was injured and that they had allowed a mole to climb high enough in their ranks to know about top secret missions. Yes cleaning house indeed.

Tsuna looked at his phone in annoyance before walking back to the school building and climbing through the window he exited. Seeing his still frozen classmates Tsuna snapped his fingers and said "Hello?"

The class snapped out of their stupor but were thrown into another one at the unfamiliar word. "Was that Italian Tsuna?" Nagisa asked.

Tsuna looked confused for a minute before realization crossed his face. "Oh yeah it was. I transition from Japanese to Italian when I talk to family and forget to switch back sometimes."

The class watched as Korosensei sped to Tsuna's side and patched up his no longer bleeding cheek. "Would you like to fill us in on why you just had an assassination attempt on your life Tsuna. I thought those were my thing." Korosensei asked as he cleaned the room.

"Sorry sensei it should be handled now but there is still a sniper nearby so you might wann-" Tsuna didn't get to finish his sentence before the sniper was laying with the rest of the bodies in the front of the class. "Never mind." Tsuna said looking at the pile.

"Continue Tsuna I would love to hear why you're so important that you have to have assassination attempts on your life." Karma spoke up from behind him with his ever present smirk.

"If I tell you and you say something to someone else I'll have to kill you." Came Tsuna's threat of silence. The class were taken aback at that, never really seeing this side of Tsuna before. They've never seen him so serious.

Once he got verbal confirmation from everyone in the room he turned to his teacher with an expectant look. "It's not like you could ever even come close to killing him." Came a comment from one of his meaner classmates.

In a burst of insane speed Tsuna shot to the front of the room and cut off three of Korosensei's arms then turned around with his dying will flame burning on his forehead and said "I'm in the Mafia. The next leader of the biggest famiglias in the world to be exact, meaning I have a target on my head larger than Korosensei. He only has this class to worry about and the government. I have half the world to deal with coming after me including the government. Going this long without some kind of protection is astonishing to me but that seems to be changing soon.

"Thats the fourth assassination attempt on my life in the past six months so I know how to deal with them but it was the first time I've been without my guardians." The class was silent for several minutes after Tsuna's speech and they finally snapped out of their musings by Karasuma slamming the door open gasping.

The fire on Tsuna's forehead extinguished instantly and he transformed back into his meek self, although not nearly as pathetic now. "What the hell just happened!" Karasuma demanded, his gun hanging loosely from his hand noticing the threat neutralized at the front of the class.

"Tsuna just kicked all these guys' asses!" Terasaka yelled, banging his fist on his desk followed by other students exclamations of how awesome it was. Karasuma looked at the shy student with a bandage on his cheek with curiosity and confusion. Curious as to how such a pathetic looking student could dispatch five armed enemies with only a cut on his cheek with little other indication he had been in a fight recently. Though, given his history with the government, he knew looks could be deceiving.


	2. Chapter 2

The following days went by as normal though Tsuna noticed he did not get picked on at all, minus the friendly jabs Karma usually took at him. In fact it seemed everybody was keeping their distance from him and Tsuna didn't know how to feel about that. Of course he'd been isolated from his peers for many years in his childhood but he never had as many friends as he did now.

By the time first block rolled around Monday Tsuna was having a crappy day. He woke up late, he had no breakfast, it was raining and his umbrella flew away soaking him instantly with still a ways away from school, not to mention the muddy mountain he had to practically parkour up. When he arrived in class muddy soaking and depressed the class looked on in shock and pity, it didn't help the fact that the only person that could get him dry and clean in a matter of seconds wasn't even there yet.

As soon as the bell rang Korosensei flew through the door with his ever present smile, somehow that smile seemed to dim when his gaze fell upon the shaking brown mop in a chair. Korosensei was going to ask Tsuna what happened but was cut off by the door slamming open showing a silver haired delinquent and tall black haired swordsman.

"Tsuna!" The silver haired delinquent yelled making everybody snap their heads to Tsuna and Tsuna snap his up. He smiled seeing his two best friends standing there smiling but their smiles quickly faded when they saw what condition he was in.

"Tsuna?" Yamamoto questioned, his smile straining so hard Tsuna wanted to look away.

"Oh what did they do to you tenth!? I shall blow them up in your honor!" Some eyebrows raised at his exclamation and fear after realizing he could do just that when he pulled dynamite out of who-knows-where.

"Ah Gokudera you really don't need to do that it's fine. Just a bit of bad luck but it just got better." Tsunas radiant smile stopped Gokudera in his tracks, along with everyone else present never seeing Tsuna smile so full of happiness and warmth. Gokudera rushed forward and wrapped his arms around his boss not even thinking about the cold seeping into his bones. They were both scooped up in the arms of a cheerful Yamamoto.

"Don't touch me you baseball idiot!" Gokudera was quick to voice his protest but Tsuna just sighed, already used to and expecting this.

"Ma ma don't be like that Hayato it's just a group hug. We haven't seen Tsuna in two months I'm sure he doesn't mind. Beside, the other guardians told us to give him a hug, well most of them." Yamamoto put the pair down and as soon as Gokudera let go, much to a cold Tsuna's chagrin Tsuna sneezed.

Both guardians looked at their friend and killing intent swirled around the room instantly drawing the attention of everyone present, including a newly arrived Karasuma and Bitch-sensei. Korosensei looked on in interest never expecting such an aura from teenagers, and if Tsuna really was their boss Korosensei wouldn't be surprised if Tsuna could create a much more intimidating aura. "I see you have been neglecting Tsuna. That's a big mistake." Yamamoto turned serious as he started to pull his sword out of his sheath much to everyone's surprise.

"I will not let this stand any longer. Tenth is sick now because of you all so you must pay." Gokudera was quick to pull his dynamite but they were both stopped by a shivering Tsuna placing his hand on their backs and sending harmonization Flames through the contact.

"Guys stop. It's not their fault, I woke up late and my umbrella flew away. Really I'm fine I just need to dry off." Needing no more inventive Korosensei was quick to go to his student only to be stopped by a sword coated in blue flames at his neck.

"Get any closer to Tsuna and I'll kill you." Yamamoto's attitude took a 180 and everybody held their breath never seeing anything like it before. Their teacher was being held back from helping one of his students and his face turned red showing he was angry.

"Stop it Yamamoto. If you move the slightest inch you'll cut me and I know you don't wanna do that." Tsuna put himself in front of Yamamoto's sword replacing his teacher knowing Yamamoto would never even think about hurting him but would have no qualms about doing it to an alien octopus who could hurt his friend.

Yamamoto slowly lowered his sword putting it back in the sheath on his back looking upset, a stark difference to what he looked like when he first arrived. Korosensei took that as his cue to change and dry his student off so he didn't get sicker than he was already. When he was finished Tsuna went to his shocked friends and said. "We have a lot to discuss. Where did Reborn put you?"

"Beside you." Gokudera responded. Korosensei raised an eyebrow at hearing Reborn's name again much less coming from a bunch of middle school students. Though, he guessed, if these kids were associated with the Mafia they would know about Rebron. He hadn't known that Reborn had taken on another student after the clutz Dino was. Korosensei knew from then on that he would have to keep a close eye on his student, especially if he was trained by Reborn then Korosensei had no doubt that this student could eventually kill him.


	3. Chapter 3

The next few weeks were filled with assassination attempts on both Korosensei and Tsuna, much to the chagrin of both his guardians and his teacher. Neither parties with attempts on their lives got seriously injured aside from the occasional bruise or scratch, well in Tsuna's case at least.

The class was adjusting well to the new transfer students Yamamoto became a quick favorite amongst the athletes and Gokudera amongst the more delinquent like students, especially Karma.

The class had also gotten used to the assassination attempts meant for Tsuna as they had become quite frequent. Korosensei was quick to defeat several of the would be assassins usually leaving a couple for Tsuna's guardians to take care of.

It was a normal day in class when everything went to shit. They were learning something in math Tsuna didn't really care about or pay attention too. His hyper intuition had been acting up all day and Tsuna was waiting for the other shoe to drop.

And boy did the shoe drop, it dropped in the form of a bullet. Not aimed at him but aimed at his blue haired friend in front of him. Tsuna had been tense the whole day, not going unnoticed by his guardians who in return were tense so when Tsuna grabbed the smaller boys collar and flung him back into Karma his guardians were ready.

Tsuna's flames quickly made an appearance on his forehead and turned the mittens on his hands into his Vongola Gloves. Korosensei quickly went on the defensive, ready to give whoever shot at his students a beating they wouldn't soon forget, but was stopped by his composed student.

"Not this one Korosensei. This ones mine. Someone needs to be set an example of or else people will continue coming after my friends. Let's go." Tsuna put his hands by his sides and quickly made his way to the sky quickly followed on the ground by his friends.

The class was left stunned at Tsuna's exclamation as Nagisa tried to make his way off Karma and his tipped over desk. Nobody was as surprised as the pair at Tsuna's strength. Not only did he lift Nagisa straight out of his chair but he also flung him halfway across the room with one arm.

The class ran to the window Tsuna broke and watched their friend fly and fight with such a cold calculation they were in wonder. They continued to watch until Karasuma busted in with Bitch-sensei a few seconds later only for the pair to stare in shock at the flying student. "It can't be." Karasuma stated in disbelief.

"Oh it can be." The blonde seductress confirmed at seeing those flames having seduced several with the same.

"How did I not notice. Did you know Korosensei?" Karasuma grunted in the yellow octopuses direction. Korosensei nodded which was met with Karasuma next question. "Why?"

"Omertà." Was all Korosensei said and Karasuma understood everything he needed to. The class continued to watch Tsuna fly around the mountain looking for anymore assassins.

When Tsuna flew back into the classroom beside his guardians he turned to his teacher and said "If you'll excuse us sensei we will be making an unplanned trip back to Italy to... deal with a few problems. I should be back in a few short weeks." Tsuna left no other warning and said no goodbyes before he was flying out of the room with his guardians flying all the way to the airport where they got on the Vongola private jet immediately heading to clean house under his supervision.

It wasn't that he didn't trust his tutor, quite the opposite actually, he had all the reason in the world to trust Reborn but it was obvious the leak was coming from somewhere else inside his Family. When Tsuna and his guardians arrived at the mansion with no prior knowledge everyone was surprised, especially when he came back angry.

"Tsuna?" Reborn questioned when he called a family meeting in his office having been back not even an hour.

"I've had several attempts on my life the past few weeks and they're getting quite troublesome and even more annoying. I trust you cleaned house Reborn but it's obvious the leak is coming from somewhere else in Vongola seeing as the attempt yesterday was after you cleaned house."

To say Reborn and Tsuna's guardians were furious would be an understatement. They had no idea Tsuna had had so many people try and kill him while they weren't all there to protect him. Reborn murderous aura filled the room and nobody, sans Tsuna maybe, were spared its effects.

"Be assured my dame student this will not happen again." Reborn growled out only to be stopped by Tsuna.

"That's where you're wrong Reborn. No matter how well you clean house there will always be defectors and traitors we can only delay the inevitable. We will go about his from a different angle. We will be discreet and precise, nobody besides this room will know what is going on. Not even the Varia for their tactics are far to loud and showy for this. We will have three weeks to do this because I have exams in four and I doubt the yellow octopus will like it if I miss exams." With that everybody left and went to do their assignments.

Mukuro and Chrome would secretly tap into the minds of everyone in the mansion and read their thoughts deciding on if they seem traitorous. If Tsuna left it up to Mukuro alone he had no doubt half of Vongola would be "traitorous" so he assigned a very much more morally correct Chrome to help with the job.

Hibari was assigned to take the ones Mukuro, Chrome, and Tsuna deemed to be traitorous and interrogate them. If they truly did turn out to be traitors Tsuna gave Hibari free reign in the punishment of said traitors. Those that were not traitors but had been suspected had their minds wiped of either the event or of the Mafia in total.

Tsuna would allow no one to betray him or his guardians.

By the time the third week was up several people had been cleansed from Vongola history and the minds of several Vongola staff wiped. When Tsuna, Gokudera, and Yamamoto returned to Japan they were just in time for review week.

The class erupted in mutters and yells as the three most mysterious students finally returned from three week long 'vacation'. Tsuna's friends were ecstatic to have him back finally after forever but felt bad because they came right before exams meaning no relaxing.

"I assume you have sorted everything out Tsuna-San?" Korosensei asked as his students took a seat.

"Everything should be fine now sensei. We only have a few more weeks to kill you don't we?" Tsuna rebutted.

Korosensei cackled "Yes. Yes you do so you better not slack off just because its exam time I expect Assassination at least three times a week. And well thought out ones too!"

Glad that the attention was finally drawn from him and his guardians Tsuna was glad for the normalcy he had regained. It was sad, he thought, that a class that has a teacher as an octopus that wants to be killed before he blows up the earth was normal.

"So what happened to you guys. You just disappeared for almost a month then come back out of the blue like nothing happened?" Terasaka demanded.

Tsuna looked up from his lunch with his ever present smile and said morbidly making the others around him shiver and Karma chuckle in delight. "I took out the trash." Tsuna turned back to his bento and popped a rice ball in his mouth.

Right as he was swallowing Tsuna got hit on the back of the head by a flying projectile with perfect aim. Tsuna choked making Gokudera freak out and Yamamoto laugh when he saw what hit Tsuna.

The others looked on in shock and amusement at the scene when a man with a murderous aura emerged from the tree line with a deadly smirk. "Omnivore" the scary man said making Tsuna pale.

The class was instantly on high alert, anything that scared Tsuna scared them. "H-Hibari? What are you doing here?" Tsuna saw the look in his eyes and sighed "You wanna fight him don't you?"

"Very much so." He confessed looking excited for the challenge ahead.

Tsuna handed him his tonfa back and said "Well he's on lunch break right now he should be back in a few minutes."

"Hn" was the reply making Tsuna roll his eyes and continue to eat his bento.

"Tsuna?" Came from the startled Nagisa "Who is that?"

"He's our friend!" Tsuna sighed when Yamamoto laughed out an answer that got the baseball player a knock to his head courtesy of their 'friend'.

"Speak for yourself baseball idiot!" Gokudera yelled. Hibari hit Gokudera as well making both of them sit quietly much to the amusement of Tsuna and the rest of the class. Yamamoto smiled as he always did while Gokudera pouted.

"He's quick Hibari. Quicker than me," Tsuna advised "this isn't going to be like fighting Dino-san."

Tsuna didn't get much in the way of responses but he never thought he would so he continued to eat his lunch.

When Korosensei showed up a few minutes later he noticed the new addition to his class and seemingly teleported right over. "Hello and who might you be?" Korosensei politely asked.

Hibari didn't grace his question with a response instead he settled into a battle stance and readied his tonfa's. "He wants to fight you," Tsuna cleared up as he continued to eat his lunch.

"Should I?" Korosensei asked his student.

"He's the strongest of my guardians and if you don't fight him he's going to want to fight me." Tsuna informed and he ignored Gokudera's indignant squak of rage.

Korosensei nodded and said "After school it is then. Right now we have classes to attend." With that Hibari scowled a little at his delayed fun but stalked off nonetheless, he didn't come here to learn he came here to fight and wouldn't waste his time with stupid herbivores.

* * *

When school came to an end the whole class gathered to see the fight. Hibari was already there and waiting with a blood thirsty smirk on his face at the upcoming challenge. If the octopus was a challenge for the omnivore then it should be plenty of fun for him.

Hibari was readying his fighting stance and preparing his tonfas when the yellow octopus held out a tentacle. "I will fight you on one condition." Hibari raised his eyebrow intrigued and fell out of his fighting stance giving his attention to the alien in front of him.

"If you win you can come and fight me anytime not during class time," Hibari's smirk got bloodier "but if I win you have to become my student."

Tsuna and his guardians choked on their saliva at the conditions as the class gawked in terror, all except Karma for he was all too thrilled to have someone as blood thirsty as him in class. Tsuna quickly regained his senses when he saw Hibari's face "Sorry Korosensei I can't agree with that. Hibari is currently needed elsewhere, he only came to fight you and leave. It seems he needs an itch scratched that nobody else is willing to scratch for him."

Hibari scowled at the implications but is satisfied by what the omnivore said. "Very well," Korosensei agreed then turned to the boy in the fighting stance in front of him "all you have to do is destroy four of my tentacles and you win."

Hibari smirked in answer and shot forward only to have the yellow alien disappear in a blur and reappear behind him with a forceful strike to his back that made him stumble. Quickly regaining his balance Hibari spun to his side to block a tentacle coming at his face. When the tentacle encountered the thin layer of cloud flames coating the tonfa it slowly melted only to grow back seconds later.

Hibari smirked, this was going to be fun, it seems like it's finally time for him to let go and lose control since it's clear this octopus was strong enough to take it. Tsuna saw the look in his clouds eyes and a pit of dread opened up in his stomach making him feel heavy.

"Get back!" He yelled to the students around startling them out of their trance from the fighting. They looked at him with questioning eyes only to see an orange hue enter his narrowed eyes.

"Get inside," He said without taking his eyes from his blood thirsty guardian "you can finish watching from the windows but it's too dangerous to be out here."

There were protests and questions thrown at him before he turned to them and his eyes glowed a little fiercer "Go."

None of them had the courage to disobey him as they quickly rushed to the class to miss as little of the fight as possible. It seems something had snapped in Hibari from the few seconds they were outside to watching in the window. He was absolutely vicious now not giving Korosensei even a small break between attacks.

Hibari had succeeded in taking out the required four tentacles with his last attack where he faked Korosensei out and quickly got behind the octopus to slice off two tentacles from either side ending the fight.

Korosensei seemed slightly confused as to what had happened but he didn't have much time to dwell on it before Hibari was attacking him again. The attack didn't last long, however, as Tsuna put a stop to it.

"Hibari!" The boy snapped in the most commanding tone any of them had ever heard as he rushed forward with the help of his flames. He got in between his cloud and teacher staring the former down and grabbed the tonfa that was ready to strike him before saying "Enough Hibari. You came here to fight, you for your fight, now go back to Italy. He isn't yours to kill and now you can fight him anytime."

Tsuna's eyes glowed orange with power and slight anger at his guardians conduct. They had a tense stare down for a few seconds before Hibari finally ceded with a 'hn' before he ripped his tonfa from the omnivores grasp and stalked off.

Tsuna let out a huff as he watched his guardian walk away and turned to his teacher no trace of the intimidating aura from just seconds ago, "Sorry Korosensei," He said as he chuckled and scratched the back of his head "Hibari gets like that sometimes when he gets too excited. And expect many fights with him from now on he's quite the addict."

Tsuna dodged a flying tonfa and let it hit the school behind him embedding itself in the bricks with a web of cracks coming from it. The students, who had come out after they saw the stand off between the two end, stared in shock and horror at the tonfa buried in their school wall.

Some of them tried to get the tonfa from the wall but couldn't even make it budge when Yamamoto came from behind them and pulled it out with ease then, in the words of most of the students, yeeted it back into the woods.

"Why did you make us go inside Tsuna?" Nagisa asked when he turned from their teacher to face them.

"Because when Hibari loses control people lose their lives." Came Tsuna's simple yet terrifying answer.

"Is he really that strong?" One of the girls at the back of the group asked.

"He took off four of Korosensei's tentacles in less than three minutes and still wanted more. So yes, he is that strong."

"Then why does he listen to you?" Karma asked intrigued.

Tsuna smirked "Because I'm stronger," he said making the other students gawk "and I have admittedly helped feed his addiction." He finished with a nervous chuckle.

* * *

 **Sorry for such a long wait guys so here's a long chapter to try and make up for all the time lost.**


End file.
